Binder, negative plate and lithium ion battery

Abstract

The invention provides a binder, a negative plate and a lithium ion battery. The binder comprises a C5-C10 alkyl acrylate structural unit, a polar structural unit and a hydroxyl-containing polyester structural unit, and the glass transition temperature of the binder is 73-93 DEG C. According to the binder, the binding power between an electrode material and a current collector and the dispersity of the carbon nanotubes in slurry can be remarkably improved, so that the rate capability and the cycling stability of the battery are effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to a binder, in particular to a binder, a negative electrode sheet and a lithium ion battery. Background Art

[0002] Lithium-ion batteries play an important role in mobile devices, electric vehicles, and energy storage systems due to their high energy density, high operating voltage, no memory effect, and environmental friendliness. In lithium-ion batteries, binders, as key materials in battery manufacturing, have an important impact on the charge transfer efficiency and cycle life of the battery. Therefore, in lithium battery manufacturing, it is crucial to select a suitable binder.

[0003] Commonly used binders include styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), etc. However, SBR and PVA are not resistant to shearing and are prone to demulsification and floating during processing, and CMC has low mechanical strength and is not resistant to shearing. In the application of negative electrode materials, carbon nanotubes (CNTs) as a common additive or enhancer, although they have many advantages such as high specific surface area, excellent conductivity and mechanical properties, still have the problem of easy agglomeration and difficult dispersion during the pulping process, resulting in reduced conductivity of the negative electrode sheet, thereby reducing the rate performance and cycle performance of the battery. Summary of the invention

[0004] In view of the above-mentioned defects, the present invention provides a binder, which can significantly improve the peel strength between the electrode material and the current collector, and the dispersibility of carbon nanotubes in the slurry, thereby effectively improving the rate performance and cycle stability of the battery.

[0005] The present invention provides a negative electrode sheet, comprising the above-mentioned binder. Since the binder can improve the mechanical strength of the electrode sheet and the dispersibility of carbon nanotubes in the slurry, the negative electrode sheet is applied to a lithium-ion battery to effectively improve the rate performance and cycle performance of the battery.

[0006] The present invention provides a lithium ion battery. Since the lithium ion battery comprises the above-mentioned binder or the above-mentioned negative electrode sheet, the battery has higher rate performance and cycle stability.

[0007] The first aspect of the present invention provides a binder, comprising C5-C10 alkyl acrylate structural units, polar structural units and hydroxyl-containing polyester structural units; the glass transition temperature of the binder is 73°C to 93°C.

[0008] The binder as described above, wherein the weight average molecular weight of the binder is 5×10 5 g / mol~1.5×10 6 g / mol.

[0009] The binder as described above, wherein the mass percentages of the C5-C10 alkyl acrylate structural unit, the polar structural unit, and the polyester structural unit containing hydroxyl are (10% - 40%):(20% - 55%):(10% - 45%).

[0010] The binder as described above, wherein the polyester structural unit containing hydroxyl is derived from a polyester oligomer monomer containing hydroxyl, and the hydroxyl value of the polyester oligomer monomer containing hydroxyl is 50mgKOH / g - 300mgKOH / g.

[0011] The binder as described above, wherein the weight-average molecular weight of the polyester oligomer monomer containing hydroxyl is 500g / mol - 3000g / mol.

[0012] The binder as described above, wherein the acrylate structural unit includes at least one of a 5-hydroxypentyl acrylate structural unit, an isooctyl acrylate structural unit, a n-hexyl acrylate structural unit, a 2-propylheptyl acrylate structural unit, and a n-pentyl acrylate structural unit;

[0013] and / or, the polar structural unit includes at least one of an acrylamide structural unit, an acrylonitrile structural unit, and an acrylic acid structural unit;

[0014] and / or, the polyester structural unit containing hydroxyl includes at least one of a polycaprolactone diol structural unit, a poly(adipic acid-co-hexanediol) polyester structural unit, a polyethylene glycol / poly(ethylene terephthalate) copolymer structural unit, and a polyester polyol structural unit.

[0015] The binder as described above, wherein the acrylamide structural unit includes at least one of an acrylamide structural unit, an N-isopropylmethacrylamide structural unit, a methacrylamide structural unit, and an N,N-dimethylacrylamide structural unit;

[0016] and / or, the acrylonitrile structural unit includes at least one of an acrylonitrile structural unit, a methacrylonitrile structural unit, and a 2-phenylacrylonitrile structural unit;

[0017] and / or, the acrylic acid structural unit includes at least one of an acrylic acid structural unit and a methacrylic acid structural unit.

[0018] The second aspect of the present invention provides a negative electrode sheet, which includes the binder described in the first aspect.

[0019] The negative electrode sheet as described above, wherein the negative electrode sheet further includes a conductive agent; the conductive agent includes carbon nanotubes.

[0020] The third aspect of the present invention provides a lithium-ion battery, comprising the binder described in the first aspect above, or the negative electrode sheet described in the second aspect.

[0021] By making the binder include a C5-C10 alkyl acrylate monomer structural unit, a polar structural unit, and a polyester structural unit containing hydroxyl groups, and making the glass transition temperature of the binder be 73°C to 93°C, the flexibility and plasticity of the binder, as well as the adhesion and compatibility between the binder and the current collector, are improved, so that a relatively high peel strength is achieved between the binder and the current collector; at the same time, the dispersion uniformity of carbon nanotubes in solvent water can also be improved, thereby comprehensively improving the rate performance and cycle performance of the battery. Detailed Embodiments

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0023] The first aspect of the present invention provides a binder, comprising a C5-C10 alkyl acrylate structural unit, a polar structural unit, and a polyester structural unit containing hydroxyl groups; the glass transition temperature of the binder is 73°C to 93°C.

[0024] Exemplarily, the glass transition temperature of the binder can be 73°C, 75°C, 77°C, 79°C, 81°C, 83°C, 85°C, 87°C, 89°C, 91°C, 93°C, or a range composed of any two of these values.

[0025] Specifically, the above-mentioned C5-C10 alkyl acrylate structural unit is obtained by free radical polymerization of C5-C10 alkyl acrylate monomers under the action of an initiator. Similarly, the polar structural unit is obtained by free radical polymerization of allyl monomers containing at least one polar group under the action of an initiator, and the polyester structural unit containing hydroxyl groups is obtained by free radical polymerization of polyester oligomer monomers containing hydroxyl groups under the action of an initiator.

[0026] Specifically, during the preparation process, the glass transition temperature of the C5-C10 alkyl acrylate monomers and the alkenyl monomers containing at least one polar group, or the mass ratio of the C5-C10 alkyl acrylate monomers, the alkenyl monomers containing at least one polar group, and the polyester oligomer monomers containing hydroxyl groups can be controlled to further control the glass transition temperature of the binder, so that the glass transition temperature of the binder is 73°C to 93°C.

[0027] The "glass transition temperature" in the present invention refers to the temperature corresponding to the transition from the glassy state to the high elastic state, which can be obtained by differential scanning calorimetry (DSC).

[0028] The "C5-C10 alkyl acrylate monomers" in the present invention refer to alkyl acrylate monomers with 5 to 10 carbon atoms in the main chain. For example, the number of carbon atoms in the main chain is 5, 6, 7, 8, 9, or 10; among them, the "main chain" refers to the chain with the largest number of carbon atoms including the functional group.

[0029] The present invention does not specifically limit the polar groups in the "alkenyl monomers containing at least one polar group". For example, they can be hydroxyl groups, amino groups, cyano groups, carboxyl groups, etc.

[0030] The present invention does not specifically limit the sources of the C5-C10 alkyl acrylate monomers, the alkenyl monomers containing at least one polar group, and the polyester oligomer monomers containing hydroxyl groups. Commercially available products well-known to those skilled in the art or products prepared by conventional preparation methods can be used.

[0031] The present invention does not specifically limit the initiator in the above radical polymerization reaction. It can be a commonly used initiator in the art. For example, the initiator includes at least one of ammonium persulfate, potassium persulfate, sodium persulfate, ammonium sulfate, trimethylamine, benzoyl peroxide, and tert-butyl peroxide.

[0032] The present invention does not specifically limit the source of the initiator. Commercially available products well-known to those skilled in the art or products prepared by conventional preparation methods can be used.

[0033] In the present invention, by making the binder include C5-C10 alkyl acrylate structural units (derived from C5-C10 alkyl acrylate monomers), polar structural units (derived from allyl monomers containing at least one polar group), and polyester structural units containing hydroxyl groups (derived from polyester oligomer monomers containing hydroxyl groups), and controlling the glass transition temperature of the binder to be 73°C to 93°C, the flexibility and plasticity of the binder are improved, thereby enabling the electrode sheet to have better flexibility, slowing down the damage and aging of the electrode sheet; at the same time, the polar structural units, as the binder branches, contain multiple polar groups (such as hydroxyl groups, amino groups, cyano groups, carboxyl groups), which can form hydrogen bonds or other chemical bonds with the surface of the current collector, helping to improve the adhesion and compatibility between the binder and the current collector, enhancing the peel strength between the binder and the current collector, improving the phenomenon of shedding or peeling caused by the volume expansion of the electrode material during the cycling process, and enhancing the cycling performance of the battery; in addition, the polyester structural units containing hydroxyl groups can form an interpenetrating network, which can interact with carbon nanotubes, and the polyester structural units containing hydroxyl groups contain both hydrophilic groups and lipophilic groups at the same time. The lipophilic groups therein can interact with the hydrophobic groups of the carbon nanotubes, and the hydrophilic groups can interact with water molecules, thereby promoting the dispersion of carbon nanotubes in solvent water, improving the dispersion uniformity of carbon nanotubes in the slurry, and comprehensively improving the rate performance and cycling performance of the battery.

[0034] In a specific embodiment, the weight-average molecular weight of the binder is 5×10 5 g / mol to 1.5×10 6 g / mol. Specifically, by controlling the reaction temperature and reaction time in the polymerization reaction, the mass ratio of the initiator to the C5-C10 alkyl acrylate monomer, the mass ratio of the initiator to the allyl monomer containing at least one polar group, the mass ratio of the initiator to the polyester oligomer monomer containing hydroxyl groups, and the mass ratio of the C5-C10 alkyl acrylate monomer, the allyl monomer containing at least one polar group, and the polyester oligomer monomer containing hydroxyl groups, the weight-average molecular weight of the binder can be further controlled so that the weight-average molecular weight of the binder is within the range of 5×10 5 g / mol to 1.5×10 6Between g / mol. Within this range, the molecular chain length is moderate. On the one hand, it can endow the binder with good solubility and fluidity, enabling it to be evenly distributed in the slurry, forming a uniform matrix phase and enhancing the dispersion effect of carbon nanotubes. On the other hand, it can provide sufficient intermolecular forces (such as hydrogen bonds and van der Waals forces), strengthening the binding force between the binder and carbon nanotubes and other active materials, as well as the adhesion between the binder and the current collector. Additionally, it helps to improve the stability of the slurry without making it overly viscous, not only allowing for better control of the binder viscosity but also facilitating the coating and forming of the slurry. Finally, it can endow the binder with sufficient flexibility and plasticity to ensure effective interaction with the surface of carbon nanotubes, enabling the binder to form a good coating on the carbon nanotubes, thereby obtaining a stable conductive network and enhancing the electrical conductivity and mechanical stability of the electrode.

[0035] When the weight-average molecular weight of the binder is less than 5×10 5 g / mol, the following problems may occur: 1) The molecular chain length is insufficient, weakening the interaction between the binder and carbon nanotubes and other active materials. This not only leads to uneven dispersion of carbon nanotubes but also reduces the binding force of the binder, resulting in insufficient binding force between the electrode material and the current collector and easy detachment during cycling. 2) The buffer capacity of the binder against the expansion of active materials decreases, and the cycle stability may deteriorate. 3) It is difficult to stabilize the conductive network, and the rate performance and cycle performance decline.

[0036] When the weight-average molecular weight of the binder is greater than 1.5×10 6 g / mol, the following problems may occur: 1) The molecular chain length is too long, reducing the flexibility and plasticity of the binder, which may affect the mechanical properties of the electrode sheet. The electrode sheet is prone to cracking or peeling, and the cycle life is shortened. 2) The viscosity of the binder is too high, the solubility is reduced, and the dispersibility becomes poor, resulting in difficulty in fully mixing the binder with carbon nanotubes or other materials. The preparation of the slurry is difficult and it is hard to coat evenly, and defects are likely to occur during the coating process, such as uneven coating or inconsistent thickness.

[0037] Exemplarily, the weight-average molecular weight of the binder can be 5×10 5 g / mol, 6×10 5 g / mol, 7×10 5 g / mol, 8×10 5 g / mol, 9×10 5 g / mol, 1×10 6 g / mol, 1.1×10 6 g / mol, 1.2×10 6 g / mol, 1.3×10 6 g / mol, 1.4×10 6 g / mol, 1.5×106 g / mol or a range composed of any two of these values.

[0038] In a specific embodiment, the mass percentages of the C5-C10 alkyl acrylate structural unit, the polar structural unit, and the polyester structural unit containing hydroxyl groups are (10% - 40%):(20% - 55%):(10% - 45%).

[0039] Specifically, the mass percentage content of the C5-C10 alkyl acrylate structural unit in the binder is 10% - 40%, the mass percentage content of the polar structural unit is 20% - 55%, and the mass percentage content of the polyester structural unit containing hydroxyl groups is 10% - 45%.

[0040] Exemplarily, the mass percentage content of the C5-C10 alkyl acrylate structural unit in the binder can be 10%, 15%, 20%, 25%, 30%, 35%, 40% or a range composed of any two of these values; the mass percentage content of the polar structural unit can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or a range composed of any two of these values; the mass percentage content of the polyester structural unit containing hydroxyl groups can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or a range composed of any two of these values.

[0041] Specifically, by controlling the mass ratio of the C5-C10 alkyl acrylate monomer, the alkenyl monomer containing at least one polar group, and the polyester oligomer monomer containing hydroxyl groups in the polymerization reaction, the mass percentages of the C5-C10 alkyl acrylate structural unit, the polar structural unit, and the polyester structural unit containing hydroxyl groups in the binder can be made to be within (10% - 40%):(20% - 55%):(10% - 45%).

[0042] Within this range, the C5-C10 alkyl acrylate structural unit, the polar structural unit, and the polyester structural unit containing hydroxyl groups can cooperate better, further improving the flexibility and plasticity of the binder, making the electrode sheet have better flexibility. At the same time, it can further enhance the peel strength and compatibility between the binder and the current collector, improve the mechanical properties of the electrode sheet, thereby effectively avoiding the problem of active material shedding caused by the volume expansion of the electrode material during the cycling process, and even the problem of electrode sheet cracking; and making the carbon nanotubes more uniformly and stably dispersed in the slurry, solving the problems of their agglomeration and difficult dispersion, thereby effectively improving the conductivity of the electrode sheet and further enhancing the rate performance and cycling performance of the battery.

[0043] In a specific embodiment, the polyester structural unit containing hydroxyl groups is derived from a polyester oligomer monomer containing hydroxyl groups, and the hydroxyl value of the polyester oligomer monomer containing hydroxyl groups is 50 mg KOH / g to 300 mg KOH / g. Among them, the hydroxyl value (expressed in mg KOH / g) reflects the hydroxyl density of the polyester oligomer monomer containing hydroxyl groups.

[0044] When the hydroxyl value of the polyester oligomer monomer containing hydroxyl groups is within the aforementioned range, the following effects are achieved: 1) The hydroxyl value is relatively moderate, which can not only introduce flexible interactions between molecules by forming hydrogen bonds, but also adjust the glass transition temperature (Tg) of the polymer, making the binder more flexible and plastic, thereby effectively improving the mechanical properties of the electrode sheet, especially the anti-bending and anti-cracking abilities, and adapting to the volume changes of electrode expansion and contraction; 2) As a polar group, the hydroxyl group can form hydrogen bonds or chemical bonds (such as esterification or coordination) with the surface of the current collector (such as aluminum foil or copper foil), enhancing the peel strength between the binder and the current collector, effectively avoiding the shedding or peeling of the electrode material caused by volume changes during charge and discharge cycles, and improving the cycle stability of the electrode; 3) The hydroxyl group can promote the uniform dispersion of carbon nanotubes in the slurry through polar interactions (such as hydrogen bonds or van der Waals forces) with the surface of carbon nanotubes; and the hydroxyl group can also interact with water molecules to enhance the dispersion of carbon nanotubes in the aqueous slurry, which is conducive to the formation of a stable conductive network of carbon nanotubes in the electrode, improving the electrical conductivity of the electrode, and further enhancing the rate performance of the battery; 4) Avoid the increase in the hygroscopicity of the binder caused by too high hydroxyl value, effectively stabilize the viscosity of the slurry, and reduce the impact on the coating performance and the stability of the electrode sheet after drying; at the same time, it can also avoid the reaction of the binder with other components during subsequent processing or in a high-temperature environment due to too high hydroxyl value, forming an over-crosslinked structure and reducing the flexibility and plasticity of the binder; in addition, it can also have good solubility, enabling the binder to be uniformly dispersed in the slurry and improving the coating uniformity of the slurry.

[0045] Exemplarily, the hydroxyl value in the polyester oligomer monomer containing hydroxyl groups can be 50 mg KOH / g, 100 mg KOH / g, 150 mg KOH / g, 200 mg KOH / g, 250 mg KOH / g, 300 mg KOH / g or a range composed of any two of these values.

[0046] In a specific embodiment, the weight-average molecular weight of the polyester oligomer monomer containing hydroxyl groups is 500 g / mol to 3000 g / mol. Within this range, the binder can not only provide sufficient flexibility to adapt to the volume changes of the electrode, but also ensure the mechanical strength of the electrode, and at the same time enhance the interfacial adhesion between the binder and the current collector and carbon nanotubes; finally, it also helps to improve the dispersion uniformity of carbon nanotubes, form a stable conductive network, and improve the rate performance of the battery.

[0047] Exemplarily, the weight-average molecular weight of the polyester oligomer monomers containing hydroxyl groups can be 500 g / mol, 1000 g / mol, 1500 g / mol, 2000 g / mol, 2500 g / mol, 3000 g / mol, or a range composed of any two of these values.

[0048] In a specific embodiment, the binder is prepared by a method including the following process:

[0049] After the first polymerization reaction of C5-C10 alkyl acrylate monomers, an allyl monomer containing at least one polar group is added to the reaction system to initiate the second polymerization reaction. Subsequently, a polyester oligomer monomer containing hydroxyl groups is added to the reaction system to initiate the third polymerization reaction, obtaining the binder.

[0050] Specifically, C5-C10 alkyl acrylate monomers and a first initiator are mixed in deionized water to obtain a mixed system. After deoxygenating the mixed system, the first polymerization reaction is carried out. During this process, C5-C10 alkyl acrylate structural units are formed; after the reaction ends, an allyl monomer containing at least one polar group and a second initiator are added to the aforementioned reaction system to carry out the second polymerization reaction. During this process, the allyl monomer containing at least one polar group is grafted onto the aforementioned C5-C10 alkyl acrylate structural units to form polar structural units; after the reaction ends, a polyester oligomer monomer containing hydroxyl groups and a third initiator are added to the aforementioned reaction system to carry out the third polymerization reaction, and are grafted onto the aforementioned C5-C10 alkyl acrylate structural units to form polyester structural units containing hydroxyl groups, obtaining the binder.

[0051] Furthermore, the temperature of the first polymerization reaction is 50°C to 70°C, the temperature of the second polymerization reaction is 70°C to 90°C, and the temperature of the third polymerization reaction is 80°C to 100°C.

[0052] Exemplarily, the temperature of the first polymerization reaction can be 50°C, 55°C, 60°C, 65°C, 70°C, or a range composed of any two of these values; the temperature of the second polymerization reaction can be 70°C, 75°C, 80°C, 85°C, 90°C, or a range composed of any two of these values; the temperature of the third polymerization reaction can be 80°C, 85°C, 90°C, 95°C, 100°C, or a range composed of any two of these values.

[0053] The present invention does not specifically limit the reaction times of the first polymerization reaction, the second polymerization reaction, and the third polymerization reaction. Appropriate reaction times can be selected according to actual situations, as long as the glass transition temperature of the finally prepared binder is 73°C to 93°C.

[0054] In one embodiment, the reaction time of the first polymerization reaction is 3 h to 7 h. For example, the reaction time can be 3 h, 4 h, 5 h, 6 h, 7 h, or a range composed of any two of these values. The reaction time of the second polymerization reaction is 4 h to 7 h. For example, the reaction time can be 4 h, 5 h, 6 h, 7 h, or a range composed of any two of these values. The reaction time of the third polymerization reaction is 3 h to 6 h; for example, the reaction time is 3 h, 4 h, 5 h, 6 h, or a range composed of any two of these values.

[0055] The present invention does not specifically limit the mixing method, as long as the reaction materials are uniformly dispersed in the mixing system. For example, mixing can be carried out by magnetic stirring or mechanical stirring.

[0056] The present invention does not specifically limit the method of deoxygenation, as long as the active oxygen in the mixing system is completely removed. For example, nitrogen is introduced into the mixing system for deoxygenation, and the nitrogen introduction time is not less than 1 h.

[0057] The present invention does not specifically limit the types of the first initiator, the second initiator, and the third initiator, which can be the same or different. For example, the first initiator, the second initiator, and the third initiator each independently include at least one of ammonium persulfate, potassium persulfate, sodium persulfate, ammonium sulfate, trimethylamine, benzoyl peroxide, and tert-butyl peroxide.

[0058] Furthermore, the mass ratio of the C5-C10 alkyl acrylate monomer, the alkenyl monomer containing at least one polar group, and the polyester oligomer monomer containing a hydroxyl group is (10-40):(20-55):(10-45). Within this range, it helps to make the mass ratio of the C5-C10 alkyl acrylate structural unit, the polar structural unit, and the polyester structural unit containing a hydroxyl group in the binder be (10-40):(20-55):(10-45), enabling the battery to have higher rate performance and cycling performance.

[0059] Furthermore, the mass ratio of the first initiator to the C5-C10 alkyl acrylate monomer is (0.5-2.5):100, the mass ratio of the second initiator to the alkenyl monomer containing at least one polar group is (0.5-2.0):100, and the mass ratio of the third initiator to the polyester oligomer monomer containing a hydroxyl group is (0.3-1.5):100. Within this range, the reaction can proceed uniformly during the polymerization process, improving the flexibility and strength of the binder.

[0060] Exemplarily, the mass ratio of the first initiator to the C5-C10 alkyl acrylate monomer can be 0.5:100, 1.0:100, 1.5:100, 2.0:100, 2.5:100, or a range composed of any two of these ratios; the mass ratio of the second initiator to the allyl monomer containing at least one polar group can be 0.5:100, 1.0:100, 1.5:100, 2.0:100, or a range composed of any two of these ratios; the mass ratio of the third initiator to the polyester oligomer monomer containing a hydroxyl group can be 0.3:100, 0.7:100, 1.1:100, 1.5:100, or a range composed of any two of these ratios.

[0061] Furthermore, the glass transition temperature of the C5-C10 alkyl acrylate monomer is -60°C to 10°C, and the glass transition temperature of the allyl monomer containing at least one polar group is 90°C to 180°C. Within this range, it helps to make the glass transition temperature of the binder between 73°C and 93°C, improve the flexibility and plasticity of the adhesive, as well as the peel strength between the binder and the current collector, and improve the dispersion uniformity of carbon nanotubes in solvent water, thereby improving the rate performance and cycle stability of the battery.

[0062] Exemplarily, the glass transition temperature of the C5-C10 alkyl acrylate monomer can be -60°C, -50°C, -40°C, -30°C, -20°C, -10°C, 0°C, 10°C, or a range composed of any two of these values; the glass transition temperature of the allyl monomer containing at least one polar group can be 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, or a range composed of any two of these values.

[0063] The binder prepared by the above preparation method has high flexibility and plasticity, which helps to enhance the flexibility of the electrode sheet; and can make the electrode material and the current collector have a high peel strength, effectively improving the phenomenon of the electrode material peeling off from the surface of the current collector or the electrode material falling off; at the same time, it can make the carbon nanotubes uniformly disperse in the slurry to obtain a stable slurry system, thereby effectively improving the rate performance and cycle stability of the lithium-ion battery.

[0064] In a specific embodiment, the C5-C10 alkyl acrylate structural unit includes at least one of 5-hydroxypentyl acrylate structural unit, isooctyl acrylate structural unit, n-hexyl acrylate structural unit, 2-propylheptyl acrylate structural unit, and n-pentyl acrylate structural unit.

[0065] Specifically, the C5-C10 alkyl acrylate structural unit is derived from C5-C10 alkyl acrylate monomers, and the C5-C10 alkyl acrylate monomers include at least one of 5-hydroxypentyl acrylate, isooctyl acrylate, n-hexyl acrylate, 2-propylheptyl acrylate, and n-pentyl acrylate.

[0066] When the C5-C10 alkyl acrylate structural unit includes the foregoing various specific structural units at the same time, the present invention does not specifically limit the ratio between the various specific structural units.

[0067] In a specific embodiment, the polar structural unit includes at least one of acrylamide structural units, acrylonitrile structural units, and acrylic acid structural units.

[0068] Specifically, the polar structural unit is derived from allyl monomers containing at least one polar group, and the allyl monomers containing at least one polar group include at least one of acrylamide monomers, acrylonitrile monomers, and acrylic acid monomers.

[0069] When the polar structural unit includes the foregoing various specific structural units at the same time, the present invention does not specifically limit the ratio between the various specific structural units.

[0070] In a specific embodiment, the polyester structural unit containing a hydroxyl group includes at least one of a polycaprolactone diol structural unit, a poly(adipic acid-co-hexanediol) copolymer structural unit, a polyethylene glycol / polyethylene terephthalate copolymer structural unit (i.e., PEG / PET copolymer), and a polyester polyol structural unit.

[0071] Specifically, the polyester structural unit containing a hydroxyl group is derived from polyester oligomer monomers containing a hydroxyl group, and the polyester oligomer monomers containing a hydroxyl group include at least one of polycaprolactone diol, poly(adipic acid-co-hexanediol) copolymer, polyethylene glycol / polyethylene terephthalate copolymer, and polyester polyol. Among them, the polyester polyol may include at least one of adipic acid (AD) modified aliphatic polyester and 1,4-butanediol (BD) modified aliphatic polyester.

[0072] When the polyester structural unit containing a hydroxyl group includes the foregoing various specific structural units at the same time, the present invention does not specifically limit the ratio between the various specific structural units.

[0073] In a specific embodiment, the acrylamide structural unit includes at least one of an acrylamide structural unit, an N-isopropylmethacrylamide structural unit, a methacrylamide structural unit, and an N,N-dimethylacrylamide structural unit.

[0074] Specifically, the acrylamide-based structural units are derived from acrylamide-based monomers, and the acrylamide-based monomers include at least one of acrylamide, N-isopropylmethacrylamide, methacrylamide, and N,N-dimethylacrylamide.

[0075] In a specific embodiment, the acrylonitrile-based structural units include at least one of acrylonitrile structural units, methacrylonitrile structural units, and 2-phenylacrylonitrile structural units.

[0076] Specifically, the acrylonitrile-based structural units are derived from acrylonitrile-based monomers, and the acrylonitrile-based monomers include at least one of acrylonitrile, methacrylonitrile, and 2-phenylacrylonitrile.

[0077] In a specific embodiment, the acrylic acid-based structural units include at least one of acrylic acid structural units and methacrylic acid structural units.

[0078] Specifically, the acrylic acid-based structural units are derived from acrylic acid-based monomers, and the acrylic acid-based monomers include at least one of acrylic acid and methacrylic acid.

[0079] The present invention also provides a method for preparing the binder of the first aspect, comprising the following steps:

[0080] 1) At 50°C to 70°C, a first polymerization reaction occurs in a first raw material system comprising C5-C10 alkyl acrylate monomers and a first initiator to obtain a first reaction system;

[0081] 2) A second raw material system comprising allyl-based monomers containing at least one polar group and a second initiator is added to the first reaction system, and a second polymerization reaction is initiated at 70°C to 90°C to obtain a second reaction system;

[0082] 3) A third raw material system comprising polyester oligomer monomers containing hydroxyl groups and a third initiator is added to the second reaction system, and a third polymerization reaction is initiated at 80°C to 100°C to obtain the binder.

[0083] Specifically, in step 1), the raw materials comprising C5-C10 alkyl acrylate monomers and the first initiator are mixed in deionized water to obtain a first raw material system. This first raw material system is deoxygenated. After the deoxygenation is completed, the temperature of the system is raised to 50°C to 70°C for the first polymerization reaction to obtain a first reaction system. During this process, the C5-C10 alkyl acrylate monomers polymerize to obtain C5-C10 alkyl acrylate-based structural units.

[0084] The present invention does not specifically limit the type of the first initiator. For example, the first initiator defined above can be used, and details are not repeated here.

[0085] The present invention does not specifically limit the mixing method and the deoxygenation method. For example, the mixing method and the deoxygenation method defined above can be adopted, which will not be elaborated here.

[0086] The present invention does not specifically limit the heating rate, which can be adjusted according to the actual situation, as long as the temperature of the first raw material system is between 50°C and 70°C.

[0087] The present invention does not specifically limit the reaction time of the first polymerization reaction, which can be the same as the reaction time of the first polymerization reaction defined above and will not be elaborated here.

[0088] In step 2), a raw material including an allyl monomer containing at least one polar group and a second initiator is mixed to obtain a second raw material system. The second raw material system is added to the above first reaction system, and a second polymerization reaction is carried out at 70°C to 90°C to obtain a second reaction system. During this process, the allyl monomer containing at least one polar group reacts with the C5-C10 alkyl acrylate structural unit formed in step 1) and grafts onto the C5-C10 alkyl acrylate structural unit.

[0089] Further, the second raw material system is added to the first reaction system under stirring, so that the second raw material system is uniformly dispersed in the first reaction system, improving the uniformity of the reaction.

[0090] Further, the addition time of the second raw material system to the first reaction system is 1 h to 3 h. Within this range, the occurrence of violent polymerization in the polymerization reaction can be avoided. It should be noted that the addition speed is a uniform addition, and the second raw material system only needs to be added to the first reaction system within the specified time. Exemplarily, the addition time can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h or a range composed of any two of these values.

[0091] The present invention does not specifically limit the type of the second initiator. For example, the second initiator defined above can be adopted, which will not be elaborated here.

[0092] The present invention does not specifically limit the mixing method. For example, the mixing method defined above can be adopted, which will not be elaborated here.

[0093] The present invention does not specifically limit the reaction time of the second polymerization reaction, which can be the same as the reaction time of the second polymerization reaction defined above and will not be elaborated here.

[0094] In step 3), the raw materials including the polyester oligomer monomer containing hydroxyl groups and the third initiator are mixed to obtain a third raw material system. The third raw material system is added to the above-mentioned second reaction system, and a third polymerization reaction is carried out at 80°C to 100°C. During the reaction, the polyester oligomer monomer containing hydroxyl groups reacts with the C5-C10 alkyl acrylate structural unit formed in step 1) and grafts onto the C5-C10 alkyl acrylate structural unit to obtain a binder.

[0095] Further, the third raw material system is added to the second reaction system under stirring to make the third raw material system uniformly dispersed in the second reaction system, improving the uniformity of the reaction.

[0096] Further, the addition time of the third raw material system to the second reaction system is 2h to 5h. Within this range, the occurrence of violent polymerization in the polymerization reaction can be avoided. It should be noted that the addition speed is a uniform addition, and it is only necessary to add the third raw material system to the second reaction system within the specified time. Exemplarily, the addition time is 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h or the range composed of any two of these values.

[0097] The present invention does not specifically limit the type of the third initiator. For example, the third initiator defined above can be used, and details are not described herein again.

[0098] The present invention does not specifically limit the mixing method. For example, the mixing method defined above can be used, and details are not described herein again.

[0099] The present invention does not specifically limit the reaction time of the third polymerization reaction, which can be the same as the reaction time of the above-mentioned third polymerization reaction, and details are not described herein.

[0100] The present invention does not specifically limit the mass ratio of the C5-C10 alkyl acrylate monomer, the alkenyl monomer containing at least one polar group, and the polyester oligomer monomer containing hydroxyl groups, which can be the same as the above definition, and details are not described herein again.

[0101] The present invention does not specifically limit the mass ratio of the first initiator to the C5-C10 alkyl acrylate monomer, the mass ratio of the second initiator to the alkenyl monomer containing at least one polar group, and the mass ratio of the third initiator to the polyester oligomer monomer containing hydroxyl groups, which can be the same as the above definition, and details are not described herein again.

[0102] The preparation method of the binder in the present invention is as follows: First, polymerize C5-C10 alkyl acrylate monomers to obtain C5-C10 alkyl acrylate structural units. Then, add an allyl monomer containing at least one polar group to the first reaction system to react with the C5-C10 alkyl acrylate structural units. Finally, add a polyester oligomer monomer containing a hydroxyl group to the second reaction system to prepare a binder with a glass transition temperature of 73°C to 93°C. This binder has good flexibility and plasticity, which is beneficial to improving the toughness of the electrode sheet. Moreover, the polar groups contained in the binder can form hydrogen bonds or other chemical bonds with the current collector, resulting in strong adhesion between the electrode material particles and between the electrode material and the current collector, effectively improving the mechanical strength of the electrode sheet. At the same time, the polyester structural units containing hydroxyl groups can form an interpenetrating network and interact with carbon nanotubes. The hydrophilic groups and lipophilic groups therein can interact with solvent water molecules and the surface of carbon nanotubes respectively, effectively improving the dispersion uniformity of carbon nanotubes in solvent water, thereby improving the uniformity and stability of the slurry, and enabling the lithium-ion battery to have high rate performance and cycle stability.

[0103] In the second aspect of the present invention, a negative electrode sheet is provided. Since this negative electrode sheet includes the binder of the first aspect, when this negative electrode sheet is used in a lithium-ion battery, the rate performance and cycle stability of the battery can be effectively improved.

[0104] In a specific embodiment, the negative electrode sheet further includes a conductive agent; the conductive agent includes carbon nanotubes. When the conductive agent includes carbon nanotubes, the binder can greatly improve the dispersion of carbon nanotubes in the slurry system and improve the dispersion uniformity and stability of the slurry, thereby further improving the rate performance and cycle life of the lithium-ion battery.

[0105] The present invention does not specifically limit the structure of the negative electrode sheet. In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least a part of the surface of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material.

[0106] The present invention does not specifically limit the material of the negative electrode current collector, and a commonly used negative electrode current collector in the art, such as copper foil, can be selected.

[0107] In a specific embodiment, the negative electrode sheet further includes a negative electrode active material; the mass ratio of the binder, the negative electrode active material, and the carbon nanotubes is (0.8 to 3):(50 to 100):(0.01 to 0.3). Within this range, the ratio of the binder, the negative electrode active material, and the conductive agent is relatively appropriate, which can not only improve the toughness of the electrode sheet, but also enable a strong peel strength between the electrode material and the current collector, effectively improving the problems of electrode material shedding and electrode sheet cracking, and improving the cycle performance of the battery; at the same time, the carbon nanotubes can be evenly distributed in the negative electrode active layer, providing a sufficient conductive network, enabling the battery to have high rate performance and cycle performance; and the content of the negative electrode active material is not too small, ensuring that the battery has a high capacity.

[0108] In the present invention, the negative electrode sheet can be prepared by existing methods. For example, a negative electrode slurry including a binder, a negative electrode active material, and a conductive agent is coated on at least a part of the surface of the negative electrode current collector, and after rolling, drying, and slitting, the negative electrode sheet can be obtained.

[0109] The third aspect of the present invention provides a lithium-ion battery. Since the lithium-ion battery includes the binder of the first aspect or the negative electrode sheet of the second aspect, the battery has high rate performance and cycle performance.

[0110] Hereinafter, the lithium-ion battery including the binder of the present invention will be introduced in detail through specific examples.

[0111] Example 1

[0112] 1) Preparation of the binder: Add 20 parts of n-hexyl acrylate (glass transition temperature is -55°C), 0.1 part of ammonium persulfate, and deionized water to the reaction kettle to obtain the first raw material system. Introduce nitrogen to drive away the active oxygen in the reaction kettle and the pipeline, raise the temperature to 60°C for the first polymerization reaction, and the reaction time is 4h to obtain the first reaction system.

[0113] Mix 50 parts of acrylamide (glass transition temperature is 165°C) and 0.25 part of ammonium persulfate to obtain the second raw material system. Add the second raw material system to the above first reaction system, and the addition time is 2h. Carry out the second polymerization reaction at 80°C, and the reaction time is 5h to obtain the second reaction system.

[0114] Mix 30 parts of the PEG / PET copolymer (hydroxyl value is 200mgKOH / g, weight average molecular weight is 561g / mol) and 0.15 part of ammonium persulfate to obtain the third raw material system. Add the third raw material system to the above second reaction system, and the addition time is 3h. Carry out the third polymerization reaction at 90°C, and the reaction time is 5h to obtain the binder.

[0115] 2) Preparation of the negative electrode sheet:

[0116] Mix 97.49 parts of artificial graphite, 2.0 parts of the binder prepared in step 1), and 0.1 part of carbon nanotubes by mass. Stir the mixture at a low speed of 15 rpm for 15 min, then add water to obtain a mixed solution with a solid content of 55 wt%. Then, pre-stir the aforementioned mixed solution at a stirring speed of 40 rpm for 15 min, followed by kneading. After stirring at 40 rpm for 40 min, disperse at a high speed of 15 m / s for 40 min, add 0.5 part of the binder, disperse at 5 m / s for 20 min, and then discharge under vacuum to remove bubbles to obtain the negative electrode slurry. Finally, coat the negative electrode slurry on both sides of the negative electrode current collector copper foil, with a single-sided coating areal density of 90 g / m 2 , and after drying, cold pressing, slitting, and welding the tab processes, a negative electrode sheet with a tap density of 1.6 g / cm 3 is obtained.

[0117] 3) Preparation of the positive electrode sheet:

[0118] Mix the positive electrode active material NCM622 (chemical composition: Li(Ni 0.6 Co 0.2 Mn 0.2 )O2), the conductive agent carbon black (SuperP), and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 97.3:1.5:1.2, and add N-methylpyrrolidone (NMP), stir evenly to prepare the positive electrode slurry. Coat the positive electrode slurry on both surfaces of the aluminum foil, with a single-sided coating areal density of 170 g / m 2 , and after drying, rolling, slitting, and welding the tab, a positive electrode sheet with a tap density of 3.4 g / cm 3 is obtained.

[0119] 4) Preparation of the lithium-ion battery:

[0120] Wind the above-mentioned negative electrode sheet, separator (PE film), and positive electrode sheet in sequence, with the separator placed in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, to form an electrode assembly. Place the electrode assembly in the outer package, inject the electrolyte and seal it, and after processes such as formation and degassing, a secondary battery is obtained. Among them, the electrolyte includes a lithium salt, an organic solvent, and an additive. The lithium salt is lithium hexafluorophosphate (LiPF6), the organic solvent is a mixed solvent of ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and ethyl propionate (EP), the additive is vinylene carbonate (VC), and the volume ratio of EC:DEC:PC:EP is 30:30:10:30; the concentration of the lithium salt in the electrolyte is 1 mol / L, the mass percentage content of the additive in the electrolyte is 5%, and the balance is the organic solvent.

[0121] Example 2

[0122] Prepare the binder and battery according to the method of Example 1, except that:

[0123] Replace n-hexyl acrylate with 2-propylheptyl acrylate (glass transition temperature is -60 °C), replace acrylamide with methacrylamide (glass transition temperature is 118 °C), and replace the PEG / PET copolymer with polycaprolactone diol (hydroxyl content is 112 mg KOH / g, weight average molecular weight is 1000 g / mol).

[0124] Example 3

[0125] Prepare the binder and battery according to the method of Example 1, except that:

[0126] Replace n-hexyl acrylate with 5-hydroxypentyl acrylate (glass transition temperature is -18 °C), replace acrylamide with acrylonitrile (glass transition temperature is 105 °C), and replace the PEG / PET copolymer with poly(adipic acid-co-hexanediol) (hydroxyl content is 224 mg KOH / g, weight average molecular weight is 500 g / mol).

[0127] Example 4

[0128] Prepare the binder and battery according to the method of Example 1, except that:

[0129] Replace n-hexyl acrylate with 2-ethylhexyl acrylate (glass transition temperature is -65 °C), replace acrylamide with methacrylonitrile (glass transition temperature is 108 °C), and replace the PEG / PET copolymer with neopentyl glycol (NPG) and 1,4-butanediol (BD) modified SOVERMOL ® 1092 (hydroxyl content is 60 mg KOH / g, molecular weight is 2000 g / mol, from BASF) replaces the PEG / PET copolymer.

[0130] Example 5

[0131] Prepare the binder and battery according to the method of Example 1, except that:

[0132] In step 1), the addition amount of n-hexyl acrylate is adjusted to 10 parts, and the addition amount of ammonium persulfate is adjusted to 0.05 part;

[0133] In step 2), the addition amount of acrylamide is adjusted to 55 parts, and the addition amount of ammonium persulfate is adjusted to 0.275 part;

[0134] In step 3), the addition amount of the PEG / PET copolymer is adjusted to 35 parts, and the addition amount of ammonium persulfate is adjusted to 0.175 part.

[0135] Example 6

[0136] Prepare the binder and battery according to the method of Example 1, with the differences being that:

[0137] In step 1), the addition amount of n-hexyl acrylate is adjusted to 20 parts, and the addition amount of ammonium persulfate is adjusted to 0.05 part;

[0138] In step 2), the addition amount of acrylamide is adjusted to 35 parts, and the addition amount of ammonium persulfate is adjusted to 0.275 part;

[0139] In step 3), the addition amount of PEG / PET copolymer is adjusted to 45 parts, and the addition amount of ammonium persulfate is adjusted to 0.175 part.

[0140] Example 7

[0141] Prepare the binder and battery according to the method of Example 1, with the differences being that:

[0142] In step 1), the addition amount of n-hexyl acrylate is adjusted to 40 parts, and the addition amount of ammonium persulfate is adjusted to 0.2 part;

[0143] In step 2), the addition amount of acrylamide is adjusted to 20 parts, and the addition amount of ammonium persulfate is adjusted to 0.1 part;

[0144] In step 3), the addition amount of PEG / PET copolymer is adjusted to 40 parts, and the addition amount of ammonium persulfate is adjusted to 0.2 part.

[0145] Example 8

[0146] Prepare the binder and battery according to the method of Example 1, with the differences being that:

[0147] In step 1), the addition amount of n-hexyl acrylate is adjusted to 40 parts, and the addition amount of ammonium persulfate is adjusted to 0.2 part;

[0148] In step 2), the addition amount of acrylamide is adjusted to 50 parts, and the addition amount of ammonium persulfate is adjusted to 0.25 part;

[0149] In step 3), the addition amount of PEG / PET copolymer is adjusted to 10 parts, and the addition amount of ammonium persulfate is adjusted to 0.05 part.

[0150] Example 9

[0151] Prepare the binder and battery according to the method of Example 1, with the differences being that:

[0152] In step 1), the addition amount of n-hexyl acrylate is adjusted to 50 parts, and the addition amount of ammonium persulfate is adjusted to 0.25 part;

[0153] In step 2), the addition amount of acrylamide is adjusted to 10 parts, and the addition amount of ammonium persulfate is adjusted to 0.05 part;

[0154] In step 3), the addition amount of PEG / PET copolymer is adjusted to 40 parts, and the addition amount of ammonium persulfate is adjusted to 0.2 part.

[0155] Example 10

[0156] Prepare the binder and battery according to the method of Example 1, except that:

[0157] In the PEG / PET copolymer, the hydroxyl value is 40 mg KOH / g, and the weight-average molecular weight is 2850 g / mol.

[0158] Example 11

[0159] Prepare the binder and battery according to the method of Example 1, except that:

[0160] In the PEG / PET copolymer, the hydroxyl value is 350 mg KOH / g, and the weight-average molecular weight is 570 g / mol.

[0161] Example 12

[0162] Prepare the binder and battery according to the method of Example 1, except that:

[0163] The weight-average molecular weight of the PEG / PET copolymer is 400 g / mol, and the hydroxyl value is 420 mg KOH / g.

[0164] Example 13

[0165] Prepare the binder and battery according to the method of Example 1, except that:

[0166] The weight-average molecular weight of the PEG / PET copolymer is 3100 g / mol, and the hydroxyl value is 65.1 mg KOH / g.

[0167] Example 14

[0168] Prepare the binder and battery according to the method of Example 1, except that:

[0169] Replace the carbon nanotubes with graphite.

[0170] Comparative Example 1

[0171] Prepare the binder and battery according to the method of Example 1, except that:

[0172] Use methyl acrylate (glass transition temperature is 10 °C) to replace n-hexyl acrylate.

[0173] Comparative Example 2

[0174] The binder and battery were prepared according to the method of Example 1, except that:

[0175] Styrene (glass transition temperature: 100 °C) was used to replace acrylamide.

[0176] Comparative Example 3

[0177] The binder and battery were prepared according to the method of Example 1, except that:

[0178] The PEG / PET copolymer was not added and the third polymerization reaction was not carried out; that is, the binder of this comparative example was obtained after the second polymerization reaction. In the second polymerization reaction, the mass fraction of acrylamide was 80 parts, and the mass fraction of ammonium persulfate was 0.4 parts, and the others remained unchanged.

[0179] Comparative Example 4

[0180] The binder and battery were prepared according to the method of Example 1, except that:

[0181] Acrylamide was not added and the second polymerization reaction was not carried out; that is, the third polymerization reaction was directly carried out after the first polymerization reaction to obtain the binder of this comparative example; the mass fraction of the PEG / PET copolymer was 80 parts, and the mass fraction of ammonium persulfate was 0.4 parts, and the others remained unchanged.

[0182] Test Example

[0183] 1. The weight-average molecular weight and glass transition temperature of the binders prepared in the above examples and comparative examples were tested:

[0184] 1) Weight-average molecular weight

[0185] The weight-average molecular weight of the binder was tested using a gel permeation chromatograph.

[0186] 2) Glass transition temperature

[0187] The glass transition temperature (Tg) of the binders prepared in the examples and comparative examples was detected using a differential scanning calorimeter (Shanghai Qunhong Instrument and Equipment Co., Ltd., model: DSC-100). The steps included: turning on high-purity nitrogen, setting the nitrogen flow rate to 0.5 L / min to 0.6 L / min, turning on the DSC power supply, and running the desktop. Set the temperature: -60 °C, hold for 10 min, from -60 °C to 130 °C, with a heating rate of 10 K / min. After setting, place the prepared sample on the heating furnace, cover the furnace body protection cover, and add an appropriate amount of liquid nitrogen to the constant temperature tank. When the sample temperature reaches -60 °C, run the test.

[0188] The test results are shown in Table 1.

[0189] 2. The peel strength between the negative electrode active layer and the negative electrode current collector in the negative electrode sheets prepared in the above-mentioned examples and comparative examples was tested:

[0190] After the coated single-sided negative electrode sheet was compacted at 1.6 g / cm 3 The electrode sheet was cut into a length of 20 cm × width of 3 cm. A 3M double-sided tape was attached to the steel plate, and the coated side of the electrode sheet was fixed face down on the tape of the steel plate. After rolling back and forth 6 times with a 2.5 kg roller, a tensile machine with a range of 20 N was used. The upper plate clamped the copper foil side, the coating and the copper foil were torn, the speed was 50 mm / min, and the tensile test was carried out at 180 °C. The data in the stable tensile force section were recorded as the peel strength (N / m).

[0191] The test results are shown in Table 1.

[0192] 3. The rate performance and cycle performance of the lithium-ion batteries prepared in the above-mentioned examples and comparative examples were tested:

[0193] (1) Rate performance test

[0194] First step: At 25 °C, first charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, and the cut-off current is 0.05C; then discharge at a constant current of 0.2C to 3.0V, end the discharge, and leave it standing for 10 min, and record the reference discharge capacity.

[0195] Second step: Then charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, and the cut-off current is 0.05C; discharge at a constant current of 0.5C to 3.0V, end the discharge, and leave it standing for 10 min.

[0196] Third step: Then charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, and the cut-off current is 0.05C; discharge at a constant current of 1C to 3.0V, end the discharge, and leave it standing for 10 min.

[0197] Fourth step: Then charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, and the cut-off current is 0.05C; discharge at a constant current of 2C to 3.0V, end the discharge, and leave it standing for 10 min.

[0198] Fifth step: Then charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, and the cut-off current is 0.05C; discharge at a constant current of 3C to 3.0V, end the discharge, and leave it standing for 10 min.

[0199] Sixth step: Then charge at a constant current of 0.5C to 4.2V, then charge at a constant voltage of 4.2V, and the cut-off current is 0.05C; discharge at a constant current of 5C to 3.0V, end the discharge, and leave it standing for 10 min; record the 5C discharge capacity.

[0200] The capacity retention rate (%) at 5C = (5C discharge capacity / reference discharge capacity) × 100%.

[0201] (2)Cycling performance test

[0202] At 25°C, charge at a constant current of 1C until 4.5V, then charge at a constant voltage of 4.5V with a cut-off current of 0.05C; then discharge at a constant current of 1C until 3.0V, record the initial discharge capacity as Q0, cycle 500 times according to the aforementioned charge-discharge mechanism, and record the discharge capacity after 500 cycles as Q1. Then the capacity retention rate (%) of the battery after 500 cycles = (Q1 / Q0) × 100%.

[0203] The test results are shown in Table 1.

[0204] In Table 1, Tg represents the glass transition temperature of the binder, Mw represents the weight-average molecular weight of the binder, P represents the peel strength of the binder, and A represents the mass percentage of C5-C10 alkyl acrylate structural units, polar structural units, and polyester structural units containing hydroxyl groups in the binder.

[0205] Table 1

[0206]

[0207] As can be seen from Table 1, the binder provided by the present invention can significantly improve the adhesion between the electrode material and the current collector, and at the same time can improve the dispersion of carbon nanotubes in the slurry, thereby effectively improving the rate performance and cycling stability of the battery.

[0208] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An adhesive, characterized in that, It includes C5-C10 alkyl acrylate structural units, polar structural units and polyester structural units containing hydroxyl groups; the glass transition temperature of the binder is 73 °C to 93 °C.

2. The binder according to claim 1, characterized in that, The weight-average molecular weight of the binder is 5×10 5 g / mol to 1.5×10 6 g / mol.

3. The binder according to claim 1 or 2, characterized in that, The mass percentages of the C5-C10 alkyl acrylate structural units, the polar structural units and the polyester structural units containing hydroxyl groups are (10% - 40%):(20% - 55%):(10% - 45%).

4. The binder according to claim 1 or 2, characterized in that, The polyester structural units containing hydroxyl groups are derived from polyester oligomer monomers containing hydroxyl groups, and the hydroxyl value of the polyester oligomer monomers containing hydroxyl groups is 50 mgKOH / g to 300 mgKOH / g.

5. The binder according to claim 4, characterized in that, The weight average molecular weight of the polyester oligomer monomers containing hydroxyl groups is 500 g / mol to 3000 g / mol.

6. The binder according to claim 1, characterized in that, The C5-C10 alkyl acrylate structural units include at least one of 5-hydroxypentyl acrylate structural unit, isooctyl acrylate structural unit, n-hexyl acrylate structural unit, 2-propylheptyl acrylate structural unit, n-pentyl acrylate structural unit; and / or, the polar structural units include at least one of acrylamide structural units, acrylonitrile structural units, acrylic acid structural units; and / or, the polyester structural units containing hydroxyl groups include at least one of polycaprolactone diol structural units, poly(adipic acid-co-hexanediol) copolymer structural units, polyethylene glycol / poly(ethylene terephthalate) copolymer structural units, polyester polyol structural units.

7. The binder according to claim 6, characterized in that, The acrylamide structural units include at least one of acrylamide structural unit, N-isopropylmethacrylamide structural unit, methacrylamide structural unit, N,N-dimethylacrylamide structural unit; and / or, the acrylonitrile structural units include at least one of acrylonitrile structural unit, methacrylonitrile structural unit, 2-phenylacrylonitrile structural unit; and / or, the acrylic acid structural units include at least one of acrylic acid structural unit, methacrylic acid structural unit.

8. A negative electrode sheet, characterized in that, The negative electrode sheet includes the binder according to any one of claims 1-7.

9. The negative electrode sheet according to claim 8, wherein, The negative electrode sheet further includes a conductive agent; the conductive agent includes carbon nanotubes.

10. A lithium-ion battery, characterized in that, It includes the binder according to any one of claims 1-7, or the negative electrode sheet according to claim 8 or 9.

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